At the bottom part of your "formatting" section you will see the POST TEMPLATE part:

Now PASTE the Embed code you copied awhile ago into the box provided and then click SAVE SETTINGS.

Now test your finished product by creating a new post. Take note that your newly created signature will NOT appear in your old posts. It will only appear by the time you created your new posts. If you want to attach your signature in your old posts, you have to do it manually by copying your Embed code and pasting it at the end of your every old post and then saving it. Mine looks like this: